Since the beginning of oil production from subsurface formations, the industry has been concerned with efficient control of the movement of unconsolidated formation particles, such as sand, into the wellbore. For example, such particle movement commonly occurs during production from completions in loose sandstone or following hydraulic fracture of a formation. Production of these materials causes numerous problems in the operation of oil, gas or water wells. These problems include plugging of formations, tubing and subsurface flow lines, as well as erosion of tubing, downhole equipment and surface equipment. These problems lead to high maintenance costs and unacceptable well downtime. Accordingly, numerous methods have been utilized to control the movement of unconsolidated particles during the production of fluids.
Gravel packing is one of the most common methods to prevent the production of sand. Generally, gravel packing involves placing pack sand, an aggregate or particulate material, in the annular space between the wellbore and a fluid permeable, perforated base pipe that is located adjacent to the production zone. A particular pack sand is selected to prevent the flow of formation particles therethrough, taking into consideration the characteristics of the particular reservoir. The perforated base pipe is designed to allow production fluids to flow therethrough with minimum resistance, while preventing both the pack sand and the formation particles from flowing into the production string. Gravel packing is commonly achieved by either an open hole gravel packing procedure or an internal gravel packing procedure, depending on the characteristics of the particular reservoir.
In addition to the use of a perforated base pipe and gravel packing, a sand control screen is commonly employed to control the movement of formation particles. These screens may comprise a continuous single wire wrapped around the base pipe. While this type of screen is capable of excluding even the smallest API grades of pack sand, these screens are easily damaged during handling, installation and production.
More recently, a sand control screen comprising a sand control screen jacket has been used. The screen jacket is fully formed from a single wire prior to attachment to the base pipe. Commonly, a plurality of ribs extend longitudinally along the internal surface of the screen jacket to provide strength to the wire and stand-off between the wire and the base pipe once the screen jacket is attached. In addition, some screen designs use prepacked sand confined around the perforated base pipe. These prepacked screens are constructed by fabricating the metal components, then forcing pack sand, either resin coated or uncoated, between the perforated base pipe and an inner wire screen or between an inner wire screen and an outer wire screen of a multi-layer screen.
It has been found, however, that whether single or multi-layer, conventional or prepacked, sand control screens are susceptible to erosion. This erosion process is particularly problematic during gravel packing, high rate water packing, or frac packing operations. There are three critical factors which are necessary for erosion of the sand control screen to occur, which are fines, velocity and time, that is, there must be fines traveling through the screen at a sufficient velocity for a sufficient period of time.
Fines are defined as any particle which can travel between the gaps in a sand control screen. The fines, however, must have a sufficient mass in order to cause damage. This critical mass occurs at roughly 50 microns. Also, the fines must be traveling at a high enough velocity in order to have the kinetic energy necessary to dislodge particles of metal as the fines impinge the sand control screen surface. Along with high velocity fines, the erosion process requires time in order to damage the sand control screen. In as little as two minutes, however, high velocity fines can damage a sand control screen to the extent that a typical 40/60 or 50/70 pack sand can travel through the eroded area of the sand control screen into the production string with virtually no resistance.
Sand control screens are particularly susceptible to erosion near the top of the screen due to the way sand fills a wellbore. For example, during gravel packing, sand fills the wellbore from the bottom toward the top. The fluid in the gravel pack slurry enters the pipe string through the gaps in the screen and the perforations in the base pipe. During the initial stages of gravel packing, the pressure required to pump fluid down the wellbore and up through the pipe string is relatively low due to the number of gaps in the screen which are available for fluid flow. The relatively low pressure allows sand to bridge off gaps even when the diameter of the sand is less than the width of the gaps. As the gaps near the bottom of the screen bridge off, sand forms a diagonal front which moves up the wellbore such that the sand near the screen is higher than the sand near casing. As the sand front moves up the wellbore, the pressure required to pump fluid through the system increases. Along with this increase in pressure comes an increase in the velocity of fluid traveling through the available gaps in the screen.
As the pressure increases, it becomes more difficult for sand to bridge off the gaps of the screen that are wider than the diameter of the sand. Thus, the top of the screen is most susceptible to erosion because the pressure and the velocity are greatest when the sand front approaches the top of the screen.
Therefore, a need has arisen for a sand control screen apparatus for filtering particles out of fluid produced from a wellbore that will not erode during gravel packing, high rate water packing or frac packing particularly under the critical conditions experienced near the top of the screen and that is that is capable of withstanding severe downhole conditions during installation and production.